The present invention relates generally to nonlinear light modulators and image projectors, and more particularly, to gamma correction circuits for use with nonlinear light modulators and image projectors that correct the grey scale linearity of images displayed thereby.
Gamma correction circuits are used to correct the grey scale linearity of image projectors in which they are employed. If a gamma correction circuit is not used, the grey scale linearity of the image projector is relatively poor, causing video images to look "washed out" and have poor color rendition. In particular, the gray scale is nonlinear or unbalanced, and this translates into fewer shades of gray, which may be insufficient to provide a good quality image.
Previously implemented gamma correction circuits have been based on the use of diode circuits. The diode-based circuits are generally disadvantageous because of higher voltage swings are required to compensate for diode voltage drops. This reduces bandwidth and consumes more power. There are sharp breakpoints caused by the diode-based correction circuits that result in a somewhat discontinuous transfer functions. The diode-based correction circuits are not temperature compensated. The diode voltage drop changes with temperature causing a shift in the breakpoint. The diode-based correction circuits also have a lower bandwidth if the diodes are used in a feedback configuration. Such feedback configurations may fix the temperature drift problem, but breakpoints are even sharper causing transfer functions to be discontinuous.
Previous implementations of gamma correction circuits have also employed digital techniques using A/D converters, a memory, and D/A converters. These circuits are relatively costly to build, especially if high speed and large bandwidth circuits are desired. Another problem arises when gamma correction is used in conjunction with dynamic threshold correction. Since some areas of the light valve require more light from the CRT to turn on then other areas and since the turn on point for each area on the light valve may be different due to the manufacturing process, threshold correction is required for various areas on this input surface of the light valve. However, when different threshold biases are used, the various areas of the liquid crystal light valve operate on different parts of the CRT drive curve. Since these points are redefined as zero or the black level then the gamma correction will be offset from the point where it should start. This can cause degradation of the projector output. It would be advantageous to account for or correct the effect of the threshold correction voltage on the gamma correction circuitry.
Consequently, it would be an improvement in the art to have an image light projector employing a gamma correction circuit that eliminates the above-mentioned problems associated with conventional diode-based and digital gamma correction circuits.